The objective of this proposal is to determine how extracellular lipoprotein bound cholesterol enters adrenal steroid producing cells. Three aspects of the problem will be examined: cholesterol membrane translocation; lipoprotein and apoprotein binding to cell membranes and intracellular cholesterol; and the effects of extracellular lipoproteins on intracellular cholesterol synthesis and cholesterol ester hydrolysis. Preliminary studies suggest that rat adrenal tissue accumulates HDL cholesterol in response to ACTH stimulation and that apoA-I participates in the uptake phenomenon. In addition, a reversible binding site for 125I-apo-AI has been observed. The current proposal is aimed at obtaining a better understanding of these processes and their effect on adrenal cholesterol metabolism. In order to understand the mechanism of uptake, we will examine net cholesterol flux from HDL into rat adrenal cells in suspensions or adrenal tumor cells in culture. The effects of ACTH as well as added Apo-A-I or Apo A-II on this process will be examined. Uptake of iodinated HDL and HDL labeled with 32P-lecithin will also be examined. Secondly the binding of 125I Apo A-I to rat adrenal tissue will be further characterized by determining the subcellular location of binding as well as the effects of pH, ionic strength, and chemical modification of apo A-I on the process. Finally the effects of extracellular lipoproteins on adrenal cholesterol synthesis and cholesterol esterase activity will be examined.